1. Field of the Invention
The present invention relates to an electrophotographic photoconductor, and more particularly to an electrophotographic photoconductor in which a surface top layer thereof has a specific oxygen transmission coefficient and contains a charge transporting material with a specific charge mobility.
2. Discussion of Background
Various inorganic materials such as Se, CdS and ZnO are conventionally employed as photoconductive materials for an electrophotographic photoconductor. However, because of poor photosensitivity, low thermal stability and toxicity of the above-mentioned inorganic materials, electrophotographic photoconductors employing organic photoconductive materials have been actively developed recent years, and in fact, a variety of organic photoconductors are applied to the commercially available copying machine and printer.
According to the Carlson process, the electrophotographic photoconductor is repeatedly subjected to a cycle of charging, exposure, development, transfer, quenching and cleaning in order to fulfill its functions. The photoconductor is therefore required to have high durability to constantly produce high quality images.
The organic photoconductor is required to have high durability in terms of its electrostatic properties in order to prevent the photosensitivity and charging characteristics from decreasing, the residual potential from increasing, and the image blur and toner deposition of background from occurring, as well as in terms of its mechanical properties in order to protect the surface of the photoconductor from wear and scratching.
To increase the mechanical durability of the photoconductor, a binder resin with high wear resistance has been studied, and the electrophotographic process has been reviewed so as to reduce the wear of the photoconductor.
On the other hand, it is known that the electrostatic properties of the photoconductor are decreased by the deposition of an oxidizing material such as ozone or No.sub.x generated by corona charging on the surface of the photoconductor, and the deterioration of a charge transporting material for use in the photoconductor. Due to the deposition of the oxidizing material on the surface of the photoconductor and deterioration of the charge transporting material, the surface resistivity of the photoconductor is decreased, thereby causing the blurring of obtained images. In addition, when the photoconductor is reused after intermission subsequent to repeated operations, white non-printed spots tend to appear in a solid image, or black stripes on a white background in the case of reversal development because of decrease of the charging properties.
The following proposals are conventionally made to solve the above-mentioned problems of defective images resulting from the deterioration of electrostatic properties of the photoconductor:
(1) Japanese Laid-Open Patent Applications 2-52373 and 3-92822.
There is proposed a method of constantly keeping the surface of a photoconductor in good condition by abrading the surface thereof.
However, this method necessitates an abrasive material, thereby increasing the cost, and the mechanical durability of the photoconductor is decreased.
(2) Japanese Laid-Open Patent Applications 2-64549, 2-64550 and 6-332216.
There is proposed a photoconductor of which photoconductive layer contains an antioxidant.
According to this method, the image blur due to the decrease of surface resistivity of the photoconductor cannot be prevented although the electrostatic durability of the photoconductor is improved.
(3) Japanese Laid-Open Patent Applications 2-67566, 2-189550 and 2-189551.
It is proposed to provide a protective layer comprising fluoroplastic particles on the photoconductive layer.
However, this method induces the decrease of photosensitivity. In addition, it is difficult to form a uniform smooth film of the protective layer, and the manufacturing cost of the photoconductor is increased.
(4) Japanese Laid-Open Patent Applications 1-284857, 1-285949 and 4-21855.
It is proposed that finely-divided particles of a lubricant be contained in a surface top layer of the photoconductor.
This method also induces the decrease of photosensitivity. It is absolutely necessary that the lubricant particles be present in the surface portion of the surface top layer, otherwise no effect will be expected.
(5) Japanese Laid-Open Patent Applications 1-191883, 1-206386 and 1-233474.
It is proposed that the photoconductor be heated to a predetermined temperature to maintain the charging characteristics and charge retention characteristics of the photoconductor in good conditions, especially under the circumstances of high temperature and humidity.
However, this method necessitates a heating member, thereby increasing the cost. There is a risk of the photoconductive layer being softened by the application of heat thereto.
As previously mentioned, the conventional proposals involve many problems, and there is no satisfactory photoconductor at the present stage.